Wireless passive mechanical vibration monitor system

This invention was made with government support under 1720415 awarded by the National Science Foundation. The government has certain rights in the invention.

The present invention relates generally to vibration sensors and in particular to a vibration sensor employing a passive wireless sender operating to boost and filter a sensed vibration signal.

Real-time vibration sensing is important in a variety of applications in industry, health care, and environmental monitoring. For example, vibration monitoring can assist in the prediction of natural disasters such as typhoons, earthquakes, and avalanche calamities, and for meteorological observations and geological surveys. Vibrations induced by human breathing and heartbeat are important vital signals for health monitoring. Vibration monitoring can also provide in-situ and non-destructive tools for diagnosing the structural health of vehicles, industrial equipment, buildings, and public infrastructures.

Contact-based vibration sensing typically uses an electronic sensor attached to the vibrating surface to provide accurate frequency and displacement measurements, for example, using strain type, piezoelectric, or electromechanical sensors. The cost of such sensors, including their supporting electronics can be prohibitively high especially if multiple monitoring locations are required. In many sensor applications providing electrical power to the sensor location or by batteries can be impractical.

For this reason, there is substantial interest in non-contact vibration sensing, that can monitor multiple locations or that avoids the need for a local source of power. Such non-contact vibration sensing systems, for example, may be based on video imaging of a vibrating surface or the measurement of Doppler shift in radio waves reflected from that surface. Low sensitivity is a key limitation to many non-contact sensing systems. Camera systems require extremely high resolution to resolve low amplitude vibration information and the long wavelength of radio signals (e.g., about 30 cm for RFID frequencies and 6 to 12 cm for Wi-Fi frequencies) result in nearly undetectable Doppler phase shifts in important ranges of lower vibration frequency.

The present invention provides a passive vibration sender boosting the amplitude of the vibration and thus the ability of remote sensing systems employing cameras or changes in a radiofrequency field to detect the vibration. The passive design potentially provides a much lower cost than active electronic sensors and eliminates the need for a source of power either locally or through external power connections. Generally, the sender provides a reflector associated with or part of a mass spring system having an eigenfrequency in a vibration frequency of interest. The sender boosts the amplitude of vibration at the reflector in a frequency of interest while reducing vibration at other frequencies, thereby increasing the signal-to-noise ratio in the remote sensing signal.

More specifically, in one embodiment, the invention provides a system for remotely detecting vibration of a vibrating object and including: a source of radiated electromagnetic energy, a reflector adapted to reflect a portion of the radiated electromagnetic energy, and a mount adapt to physically couple the reflector to the vibrating object to promote vibration of the reflector. A receiver is also provided sensing the reflected radiated electromagnetic energy for measurement of the vibration of the reflector. Importantly, the reflector provides a coupled mass and elastic biasing producing a predetermined reflector eigenfrequency that boosts predetermined vibration frequency of the vibrating object for detection by the receiver.

It is thus a feature of at least one embodiment of the invention to greatly improve remote-sensing sensitivity using a low-cost passive vibration sender.

The source of radiated electromagnetic energy can be either an optical or radiofrequency transmitter and the receiver can be either an optical sensor or radio receiver. For these purposes, the reflector may be either an optical reflector and/or a radiofrequency reflector having a reflector dimension at least one tenth the wavelength of the electromagnetic energy.

It is thus a feature of at least one object of the embodiment of the invention to provide a reflector that can be readily detected by optical or radiofrequency remote sensors.

In one embodiment, the reflector may employ an elastic membrane held within a frame providing a vibrational boundary at the eigenfrequency.

It is thus a feature of at least one embodiment of the invention to provide an extremely low-cost reflector system for use on a vibration sensor integrating the reflector and spring element.

The elastic membrane may include at least one attached weight whose mass lowers the eigenfrequency or may provide for multiple attached weights associated with distinct eigenfrequencies.

It is thus a feature of at least one embodiment of the invention to permit multiple eigenfrequencies for sending frequency information in different frequency bands.

In this case, the receiver may provide an identification table identifying the reflector among multiple reflectors according to one of at least two distinct eigenfrequencies.

It is thus a feature of at least one embodiment of the invention to allow the deployment of multiple senders and to distinguish between the senders from vibrational energy channeled into unique eigenfrequencies.

In some embodiments, the reflector may be a metallized polymer.

It is thus a feature of at least one embodiment of the invention to boost the radio reflectivity of the membrane using a thin coating with minimal effect on the eigenfrequency.

The eigenfrequency may be at a frequency with a range of 1 to 400 Hz.

It is thus a feature of at least one embodiment of the invention to provide improved sensing at low frequencies where Doppler phasing is reduced.

The mass spring system may have a quality factor of at least 10.

It is thus a feature of at least one embodiment of the invention to permit significant energy accumulation in the resonant system boosting amplitude.

The mount may provide a base directly attaching to the vibrating object and a swivel allowing repositioning of the reflecting surface at different angles with respect to the base.

It is thus a feature of at least one embodiment of the invention to provide a flexible repositioning of the reflector for improved sensitivity both with respect to the electromagnetic sources and receivers and a major axis of vibration being detected.

The mount may be an article of clothing, a strap, or an adhesive bandage adapted to physically couple the reflector to a human vibrating object to measure physiological vibration.

It is thus a feature of at least one embodiment of the invention to provide a low-cost sender for biological applications such as heartbeat or respiration rate monitoring.

These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention.

Referring now to, a sensing systemmay measure vibrations of a vibrating objectsuch as a machine, vehicle, geological formation, public infrastructure, or the human body. In some cases the vibrating objectwill have a primary axisdepicted inas vertically oriented and being in a direction of greatest vibrational amplitude.

In the case of an inanimate vibrating object, a vibration senderis mechanically affixed to the vibrating object, for example, through a baseattached to the vibrating objectby an adhesive(shown in) or machine fasteners, or the like. The baseis designed to couple vibrations of a frequency band of interest to other parts of the vibration senderincluding a reflector.

For the purpose of remote sensing, the reflectormay be illuminated by electromagnetic radiation, for example, from an optical sourcesuch as, but not limited to, conventional room lighting or optical sources concentrating energy within infrared or ultraviolet ranges.

Alternatively, the reflectormay receive radio waves from a radio transmitter, for example, a Wi-Fi transmitter operating at a frequency band of 500 MHz or higher. The radio transmitterneed not be dedicated for the purpose of sensing but may be part of a communication network used for other purposes and providing a carrier signal that may be used for remote sensing.

Electromagnetic energy from the optical sourceand/or the radio transmitterreflects off of reflectoras the reflectorvibrates to be scattered, deflected, blocked, or refracted in whole or in part as a function of the vibration of the reflectoron a path from the optical sourceand/or the radio transmitterto an optical sensorand/or a radio receiver.

In one embodiment, the optical sensormay be a video camera capable of producing image information for monitoring displacements of the reflectoror a light intensity detector detecting fluctuations in the intensity of the received light. The radio receivermay provide standard radio reception circuitry that can monitor an amplitude or phase shift of a received carrier frequency from the radio transmitter, for example, through AM or phase shift demodulation.

Referring now to, the basemay provide a swivelwhich, in one example, may be in the form of a ball and socket allowing ready adjustment of an angular positioning of a reflectorattached to swivelwith respect to the remainder of the base. This angular adjustment, for example, may allow a surface normalof the reflectorto be moved to among different angles with respect to the primary axisof the vibration or surface to which the baseis attached. In practice, this angle may be adjusted to increase the vibrational coupling of the reflectorby increasing its alignment with primary axis, or to improve the modification of the electromagnetic signal by changing the angles of incidence and reflectance of the electromagnetic signal. The maximum sensitivity when the optical sensoris a camera may be obtained when the swivelis arranged such that the viewing angle of the video camera is parallel to the vibrating surface; however, other angles may be preferred for radio or a single photodetector detection.

The present invention is intended for remote sensing and accordingly, in some examples, the optical sensormay be separated from senderby one meter or more and the optical sourceseparated from the senderby one meter or more. Similarly, in some examples, the radio transmittermay be separated from the senderby one meter or more and the radio receiverseparated from the senderby one meter or more, also for remote sensing.

A single radio receiver/radio transmitterpair can monitor multiple sendersdistinguishing them through an identifying secondary eigenfrequency as will be disclose below. A single optical sensor/optical sourcemay monitor multiple senders, distinguishing them by spatial location.

The optical sensorand/or the radio receivermay provide outputs to a computer, for example, providing one or more processors communicating with electronic memory holding a stored programwhose operation will be described below. The computermay attach to a user terminal, for example, providing output plots or data as will be discussed below. The computerexecuting the stored programoperates to extract motion information from a camera image or fluctuating grayscale value available as changes in intensity of received light (for the optical sensor) or carrier signal strength or phase (for the radio receiver) to produce a quantitative or qualitative measurement of vibration of the vibrating object. When optical sensing is used, motion detection of the imaged surface of the reflectormay be employed or grayscale value analysis at the vibration boundary in the acquired image, the latter allowing smaller vibration movement to be detected. As will be discussed further below, the operation of the senderis such as to provide improved vibrational sensitivity over a direct sensing of the vibrating objectitself without the sender.

Referring now toand, improved vibration sensing using the vibration senderis provided by a mechanical mass and spring system incorporating the reflectorof the sender. In one case, this mass and spring system provides an elastic membraneoffering a generally planar surface when un-deflected and a restoring biasing force when deflected. The clastic membranethus provides a spring of a mass spring system whose elastic constant k is determined by membrane size, thickness, material, and tension or strain.

The elastic membraneprovides a distributed mass which is preferably augmented by a discrete weightand may be placed on the membrane at a vibrational node. The membraneand the discrete weightthus provide a mechanical system having a mechanical resonance at a defined eigenfrequency that can be set to a frequency of interest of the vibrating object. Generally, increasing the weight of the discrete weightor area of the membraneor decreasing its stiffness will lower the eigenfrequency, while decreasing the weight of the discrete weightor the area of the membrane or increasing its stiffness will increase the eigenfrequency.

In one embodiment, the elastic membranemay be a thin rubber film. Typically, but not necessarily, the membranewill have a circular area and will be attached to a corresponding circular rim. The rimis constructed of a material having substantially different dynamic properties than the membraneso as to create a vibrational stop defining the boundary conditions of the membrane. In the case of a circular membrane, the resulting system will vibrate according to the Bessel function and the modes of a drum head. When a single discrete weightis used, it may be placed at the center of the membraneat a nodal point of the lowest mode of the effective drum mode.

In one embodiment, the space under the membranemay be vented through vent holes. In an alternative embodiment, however, air trapped beneath the membranemay be used to boost the effective spring constant of the membranefor higher eigenfrequencies. Generally, the vibration of the mass spring system formed in this matter will have a relatively high quality factor (Q factor) of greater than two, and typically greater than ten, and in some cases greater than twenty.

In one embodiment, the reflectormay be provided by a thin reflective sheet separate from but attached to the membrane. Alternatively the membraneitself may be the reflector augmented by optional direct metallization of the membranethrough sputtering or the like. For optical reflectance, a localized retroreflective target material, such as glass beads or fluorescent materialssensitive to the particular frequency of electromagnetic radiation, may be attached to a nodal point (typically the discrete weight) for maximum amplitude motion. These treatments operate the signal-to-noise ratio of the optical detection by allowing for a rejection of room lighting with filters or the like.

Referring now to, the tuned mass spring system described above allows the amplitude of vibration of the membraneto greatly exceed the amplitude of vibration of the vibrating objectaccording to the quality factor Q of the mass spring system. Referring to, when the vibration of the vibrating objectincludes multiple frequencies, the quality factor of the mass spring system can also band limit the vibrationof the membrane, suppressing frequenciesof the vibrating objectoutside of a frequency of interest fof the eigenfrequency of the membrane, simplifying measurement of a frequency of interest.

Referring now to, multiple discrete weights-may be applied to a single membraneto promote multiple vibration frequency eigenfrequencies, e.g., fand f, of the membranethat are determined by the masses and locations of the discrete weights as well as the membrane properties. The relationship between the masses and the locations and the eigenfrequencies can be determined empirically and are described generally in GAO, C., Halim, D., & Rudd, C. (2018), Study of vibration absorption characteristics of membrane-type resonators with varying membrane configurations, Paper presented at 47th International Congress and Exposition on Noise Control Engineering: Impact of Noise Control Engineering, INTER-NOISE 2018, Chicago, United States.

The result is the control of two eigenfrequency frequencies fand fwhich may be independently set. This allows either the analysis of two different vibration frequencies (boosted by the sender) or the ability to encode identifying or labeling frequency information in the vibration of the membrane. This allows identification of a particular senderin a set of different sendersdistinguished by, for example, frequency fwhich may vary among senders, whereas all sendersmay optionally have equal eigenfrequencies of fproviding the vibration information being monitored. In this case, the computermay include a table matching eigenfrequencies to particular different sendersso that the additional eigenfrequency effectively provides an identifying label for the sender. The ability to monitor and identify frequency fpresumes a set of rich harmonics in the vibration of the vibrating objectsufficient to provide the necessary stimulation at the set frequency as will usually be the case.

Referring now to, in an alternative embodiment multiple different membranes-may be assembled together in a single senderto operate in parallel to define multiple eigenfrequencies f-fwhich can serve the same purpose of either allowing different simultaneous vibration frequencies to be monitored (boosted by the sender) or to allow encoding of frequency information distinguishing among senders.

Referring now to, in an alternative embodiment for animate vibrating objects, a senderof the present invention (for example, employing the second embodiment ofhaving a cylindrical base) may be attached to a flexible strapto couple the senderto a human for monitoring biological vibration. Example flexible strapsinclude a bandage with an adhesiveor a loop with elastic material adjustable with a hook and loop type fastener. Such biological vibration of interest may, for example, include but not be limited to respiration, heartbeat, or the like. In this case, the eigenfrequencies will be set to an extremely low value on the order of 1 Hz.

This ability to monitor biological processes remotely with a low-cost sensor allows a senderto be built into articles of clothing, for example, as shown in. In one embodiment clothing may include baby clothing allowing baby monitoring using the techniques described above.

Referring again to, in an alternative embodiment of the vibration sender(shown in inset in), the base′ may be an upstanding cylindrical tube attached to the vibrating objectat a lower rim and whose upper rim supports a membraneand reflector. Other than the lack of a pivot discussed above with respect to a first embodiment, this embodiment operates in the same manner as the first embodiment.

While the above description primarily relates to sub acoustic frequencies, it will be appreciated that the design principles of the invention can be applied to monitoring audio frequency and ultrasonic frequency vibration as well. The sendermay be battery-free and requires no electrical power source.